This invention relates to a safety controller capable of adding input and output points in predetermined units such as safety PLCs of the so-called building block type and safety remote IO terminals connected to a safety PLC through communications.
Safety controllers of the type capable of adding input and output points in predetermined units such as safety PLCs of the building block type and safety remote IO terminals connected to a safety PLC through communications have been known. A safety controller of this type is basically structured such that any number of input-output (IO) units may be connectable to a single IO unifying unit which is a CPU unit in the case of a safety PLC and a communication unit in the case of a safety remote IO terminal.
Several connecting structures have been proposed for connecting a desired number of IO units to a single IO unifying unit.
A back plane connecting structure is one of the known examples of such connecting structure. According to this example, connectors are provided at certain intervals on a motherboard with a bus line and each connector is connected to the connector of one of the units such that the IO unifying unit can be connected to the individual ones of the IO units.
According to another example, inter-unit connectors are used as the connecting structure. Male and female connectors are provided to one of the surfaces of the case of the IO unifying unit and both left-hand and right-hand side surfaces of the case of each of the IO units and partial buses are provided inside the IO units for connecting the connectors on the left-hand and right-hand sides. If these units are connected in a series, a continuous bus line is formed from the IO unifying unit to the series of IO units.
FIG. 10 shows the structure of a prior art safety PLC as an example of safety controller, having a single CPU unit 5 of a building block type connectable to any number of IO units 6 such as an input unit 6a and an output unit 6b. 
An internal circuit 51 is contained inside the CPU unit, and each IO unit 6 contains its internal circuit 61. Each of these internal circuits 51 and 61 is structured by a microcomputer including a microprocessor (MPU) and a memory and these microcomputers serve to realize the various functions of the CPU unit 5 and the IO units 6. The CPU unit 5 has a power-receiving terminal for receiving power from a power source 7 for internal circuits. The power thus received is distributed through power distribution lines L30, L31 and L32 to the internal circuits 51 and 61 inside each unit so as to activate them. B30, B31, B32, etc. indicate an inter-unit bus which is used for exchanging input and output data among the CPU unit 5 and each of the IO units 6.
Each IO unit 6 includes input circuits 65 if it is an input unit 6a and output circuits 66 if it is an output unit 6b. As well known to persons skilled in the art, each input circuit 65 is for creating a logical signal corresponding to the on-off condition of a corresponding external switch SW and communicating it to the associated internal circuit 61 and each output circuit 66 is for driving a corresponding load LD according to a logical signal outputted from the associated internal circuit 61.
Each IO unit 6 receives power from an IO power source 8. If the IO unit 6 is an input unit 6a, the power received from the IO power source 8 is supplied through a voltage monitoring circuit 63 and a power line shutoff circuit 62 to a power supply circuit 64 for each input channel adapted to be on-off controlled by the internal circuit 61. If the IO unit 6 is an output circuit 6b, the power received from the IO power source 8 is supplied through a voltage monitoring circuit 63 and a power line shutoff circuit 62 to the output circuit 66 of each output channel. Each output circuit 66 is on-off controlled by the internal circuit 61.
In the field of factory automation, there is a strong desire to realize an improved IO structure with a reduced control unit number (or to reduce the number of unused IO points in a IO units) and to miniaturize the IO unit itself by making it slimmer such that changes in and addition to the system can be easily effected.
In the case of a prior art safety controller as shown in FIG. 10, IO units with a large number of points may not present a problem but there are problems in miniaturizing the system and reducing the point number. For example, IO power sources 8 are connected for supplying power to external IO devices such as switches SW and loads LD, and this means that terminals are required for each IO unit 6. The user will be required to provide wires for these units 6 and this will mean an additional cost for the wiring and an increased size of the control board with a large wiring duct, contrary to the original wish for miniaturization.
Moreover, since the voltage monitoring function for the IO power source 8, components for reducing noise and fuses will have to be mounted to each of the IO units 6, it is difficult to prevent the cost and the space required for the mounting from increasing.